Differences in 906 heads....

[mopar_chuck]

I’m sure a lot of guys on here already know this but for those who don’t, I discovered something about 906 heads the other day. Both heads were taken off of the same engine but the stands that the rocker shafts bolt to are different. Recently, I purchased a set of Isky adjustable rockers off this forum for the 383 we are building for the RR. When we test fitted the rocker assembly on one head, it was very evident that the sides of the rockers were rubbing the rocker stands on the head and that they would need to be clearanced. When we tried it on the other head, there was no rubbing at all because there was clearance already from the factory casting. The tops of the rocker stands were about .125” narrower on the head that had the “cast” clearance. Here are some pics....

 

[moparedtn]

Differences in casting dates? End pedestal versus inboard one?

 

[mopar_chuck]

I did not check the casting date but did come off the same engine. The outboard ones were only clearanced towards the center of the head and other 3 were clearanced on both sides. This can be seen in the last 2 pics

 

[WileE]

Sloppy factory machining ?

 

[PRHeads]

I never paid any attention to which ones were narrow or wide.

A different letter code perhaps?
(Bottom of the head under the intake ports.)

I have some early Isky rockers, which were way too wide to use with any type of normal width hold down(I “think” they would have worked with the early heads that used the aluminum bolt on stands).
I ended up narrowing them down to be the same width as the Crane iron rockers.

 

[Cranky]

Just another thing that needs to be checked. I'm sure they used more than one casting form to pour with......just how many heads were cast at the same time is something I do not know. Also, another thing that should always be checked is the CC's on ALL of the combustion chambers. I've found that there was usually a smaller one on the end of each head I checked.

 

[mopar_chuck]

Sloppy factory machining ?

Nope, this is right in the casting

 

[mopar_chuck]

I never paid any attention to which ones were narrow or wide.

A different letter code perhaps?
(Bottom of the head under the intake ports.)

I have some early Isky rockers, which were way too wide to use with any type of normal width hold down(I “think” they would have worked with the early heads that used the aluminum bolt on stands).
I ended up narrowing them down to be the same width as the Crane iron rockers.

The Isky rockers I bought could very well be the "wide" ones that you speak of. Not sure as I have nothing else to compare them to. We considered cutting the rockers for clearance, but figured it would be best to machine the sides of the rocker stands to match the other heads. This way, everything could be swapped back and forth.

 

[mopar_chuck]

Just another thing that needs to be checked. I'm sure they used more than one casting form to pour with......just how many heads were cast at the same time is something I do not know. Also, another thing that should always be checked is the CC's on ALL of the combustion chambers. I've found that there was usually a smaller one on the end of each head I checked.

We did cc the heads and all chambers were within 1 cc of one another. They have been planned .060" and to our surprise still came out to around 84cc. I thought there would be less than this with having so much material removed....

 

[Cranky]

Rule of thumb on milling open chamber heads is .0042" equates to 1cc so you should have removed around 14cc if I did my math right. Open chamber heads are usually around 90cc to start with and never checked one that was over 92cc

 

[mopar_chuck]

So basically, there is no way that .060" was removed from the head..... Is there any reference measurement to go by?

 

[Cranky]

Do you any stock heads laying around? Never really tried to measure the heights of them but you might could check if you have a flat surface to go off of or have a large mic Thing is, with the way production tolerances were, even doing that probably won't be much use but measuring from the rocker shafts to the deck surface might help tell you something....?

 

[mopar_chuck]

I will check and report back. But if they measure 84cc then chances are they were just "cleaned up" according to what you are saying....

 

[moparedtn]

I never paid any attention to which ones were narrow or wide.

A different letter code perhaps?
(Bottom of the head under the intake ports.)

I have some early Isky rockers, which were way too wide to use with any type of normal width hold down(I “think” they would have worked with the early heads that used the aluminum bolt on stands).
I ended up narrowing them down to be the same width as the Crane iron rockers.

Apparently this was a "thing" at one time with the Isky ones. I've been reading some older posts wherein Isky is being quoted as telling customers to grind the little ears off the spacers on the shaft mounts to give more clearance to their rockers.
Huh....
If that was me, those rockers would be headed right back to Isky, pronto.

 

[66Satellite47]

I had at least a couple dozen 906 heads. The chamber CC's were generally 86 -88 CC. Never noticed a big difference in the rocker stands. The later heads were more like 90 + CC's.

 

[33 IMP]

Apparently this was a "thing" at one time with the Isky ones. I've been reading some older posts wherein Isky is being quoted as telling customers to grind the little ears off the spacers on the shaft mounts to give more clearance to their rockers.
Huh....
If that was me, those rockers would be headed right back to Isky, pronto.

Apparently I have that set of iskys, because that is exactly what was done on mine, select fit . Some hold-downs have very thin pads on one side, none on the other, some pads completely gone. I think they were made for the max wedge aluminum stands, they are thinner than the steel hold-downs.

 

[mopar_chuck]

Apparently this was a "thing" at one time with the Isky ones. I've been reading some older posts wherein Isky is being quoted as telling customers to grind the little ears off the spacers on the shaft mounts to give more clearance to their rockers.
Huh....
If that was me, those rockers would be headed right back to Isky, pronto.

Yeah it's hard to believe they would send them out this way but like most aftermarket items that I have ever bought for anything, there is usually what I call "modding the Mod." You could grind the ears off the spacers, but then I suspect that the rocker tip would not be centered on the valve stem. Funny thing is that I was told that Isky made some of the better adjustable rocker arm set ups for BB Mopars....

 

[33 IMP]

Yeah it's hard to believe they would send them out this way but like most aftermarket items that I have ever bought for anything, there is usually what I call "modding the Mod." You could grind the ears off the spacers, but then I suspect that the rocker tip would not be centered on the valve stem. Funny thing is that I was told that Isky made some of the better adjustable rocker arm set ups for BB Mopars....

Mine were ground in order to get the tip centered over the valve. Some were up to an 1/8" off.

 

[BSB67]

Apparently this was a "thing" at one time with the Isky ones. I've been reading some older posts wherein Isky is being quoted as telling customers to grind the little ears off the spacers on the shaft mounts to give more clearance to their rockers.
Huh....
If that was me, those rockers would be headed right back to Isky, pronto.

I don't think Isky has made those for 30 years. I do have a set. Ground off the hold down tab. Had to touch a few pedestals too. Perfect. More surface area to distribute the load. I like them better than the Cranes because they are wider, and the hardened pad.

They are very good rockers, and I would buy them again. Mine have been 8000 rpm. If grinding the tab is too much, just throw them away, or give the to me. Then go buy something better.

 

[66Satellite47]

I ran the Isky ductile rockers in several motors. I think they were the best. Like anything in a performance motor, modifications are needed. Adjusting the side clearance is one of those things. I used solid spacers, not springs & shaved the hold down tabs as needed. Just what we need to do for a performance motor. This is still true with the modern rocker setup's.